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Module 1 · Lesson 1.1

Why concrete needs reinforcement

The one property that decides everything about how concrete is used.

Why this matters

Concrete is cheap, mouldable, fire-resistant, durable and enormously strong in compression. It is also close to useless in tension — and almost every structural member has tension somewhere in it. Everything else in this course follows from how that one problem is solved.

By the end of this lesson you should be able to

  • State the approximate ratio of concrete's tensile to compressive strength
  • Explain why concrete's tensile strength is not merely low but unreliable
  • Say where reinforcement must go, and why
  • Describe what composite action requires

What you should already know

  • Bending moments and where tension occurs in a beam (Structural Analysis Fundamentals, Module 3)
  • Direct stress and strain (Structural Analysis Fundamentals, Module 7)
  • Bending stress and the neutral axis (Structural Analysis Fundamentals, Module 9)

Take a plain concrete beam and load it. Bending puts the top in compression and the bottom in tension — exactly as it would in a steel beam.

The compression side is fine. Concrete of an ordinary structural grade will take 30 N/mm² without distress.

The tension side is the problem. The same concrete has a tensile strength of roughly 3 N/mm², about a tenth of its compressive strength. So the bottom face reaches its limit long before the top face is anywhere near its own, and the beam fails at a small fraction of the load its compression side could have carried.

Worse, that 3 N/mm² is not a number you can rely on. Concrete fails in tension from the largest flaw present — a shrinkage crack, a void, a plane of weakness under a piece of aggregate. Test twenty nominally identical specimens in compression and the results cluster; test them in tension and they scatter. So tensile strength is not merely low: it is variable in a way that cannot be designed around.

Elevation of a plain concrete beam on two supports under downward load, with a crack starting at the bottom face near mid-span and running up through most of the depth.vertical: flexuralinclined: shearinclined: shearplain concrete: the first crack runs through and the beam falls apart
With no reinforcement, the first flexural crack has nothing to cross it. The beam fails suddenly, at a load governed by the weakest flaw on the tension face.

Predict first

A plain concrete beam fails in bending. Where does the failure begin, and why?

The solution is to put a material that is good in tension exactly where the concrete is not: steel reinforcement, near the tension face.

For that to work, four things have to be true, and it is worth noticing how fortunate it is that they all are.

  1. 1.The steel must be strong in tension. Reinforcement has a characteristic yield strength of around 500 N/mm² — over a hundred times the concrete's tensile strength.
  2. 2.The two must act together. Steel embedded in concrete is gripped by it, through chemical adhesion, friction and, dominantly, the mechanical bearing of the ribs rolled onto the bar. This is bond, and it is what makes the pair behave as one member rather than two.
  3. 3.They must expand and contract alike. Steel and concrete have almost the same coefficient of thermal expansion, around 10 to 12 × 10⁻⁶ per °C. Had they differed appreciably, ordinary temperature change would have destroyed the bond and the idea would never have worked.
  4. 4.The concrete must protect the steel. Steel corrodes; the alkaline environment inside concrete passivates it, and the cover keeps corrosive agents away. Concrete also insulates the steel in a fire, which matters because steel loses strength rapidly with temperature.

The partnership is genuinely complementary. Each material does what the other cannot, and each protects the other from its own weakness.

Elevation of a reinforced concrete beam on two supports, with several fine cracks rising from the bottom face near mid-span and a reinforcing bar running along the bottom, crossing every crack.vertical: flexuralinclined: shearinclined: shearcracks open on the tension face; the bars carry the tension across them
With reinforcement the beam still cracks — but the bars cross the cracks and carry the tension, so the member goes on carrying load. Many fine cracks form instead of one wide one.

Worked example

How much load does the tensile strength actually buy?

Given

  • Plain concrete beam, rectangular, 300 mm wide × 550 mm deep, spanning 5.00 m simply supported
  • Concrete of class C30/37, so mean tensile strength fctm ≈ 2.9 N/mm²
  • Compare with the same section reinforced with 3 no. 25 mm bars at an effective depth of 490 mm

Find

The moment at which the plain section cracks, and the moment the reinforced section can resist.

    Practice

    A plain concrete section is 300 mm wide and 550 mm deep. With a mean tensile strength of 2.90 N/mm², at what bending moment does it crack, in kNm?

    Practice

    For a concrete of class C30/37 with fck = 30 N/mm², the mean tensile strength is fctm = 0.30 fck^(2/3). What is fctm, in N/mm²?

    Practice

    Steel has a coefficient of thermal expansion of about 12 × 10⁻⁶ per °C and concrete about 10 × 10⁻⁶ per °C. Over a 30 °C temperature rise, what is the difference in free strain between them, in microstrain?

    Summary

    • Concrete's tensile strength is roughly a tenth of its compressive strength, and far more variable
    • Reinforcement goes where the concrete would otherwise be in tension
    • Bond, matched thermal expansion, alkaline protection and fire cover make the partnership work
    • At the ultimate limit state, concrete in tension is assumed to carry nothing
    • Its tensile strength still matters — for when cracking starts, and how wide cracks become
    Progress is kept in this browser only.

    This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint